A QUASI-CRYSTALLINE MODEL OF DIFFUSION IN TERNARY LIQUID SYSTEMS

A quasi-crystalline model of a dilute ternary liquid system is proposed in which diffusion occurs by an interchange of species on neighboring lattice sites. Nonelectrolyte molecules are assumed to interchange with any type of diffusing species present in the system. In ternary nonelectrolyte systems relatively large cross-effects are associated with the mutual interchange of the two dilute solute species. The symmetry of the L-matrix arises directly through the application of detailed balance (microscopic reversibility) to this unit process.In electrolyte solutions, the only significant contribution to the ionic fluxes is expected to arise through interchange of ions with uncharged molecules. For diffusion in an aqueous solution of two strong electrolytes with a common ion, large off-diagonal L-coefficients result from the transformation of the lattice-referred diagonal diffusion matrix, written in terms of ionic fluxes and forces, to a form which relates the two independent salt fluxes and forces. Symmetry of the L-matrix in this case arises indirectly through the application of detailed balance to the ion-solvent interchanges. In aqueous electrolyte solutions the dilute solution ternary L-coefficients can be calculated from the independent individual ionic mobilities at infinite dilution, and these are found to be in excellent agreement with the experimental values for the system NaCl–KCl–H2O. The application of the model to binary electrolyte solutions at infinite dilution correctly yields the Nernst equation.The model is extended to solutions of two electrolytes with one or both incompletely ionized and to solutions containing a nonelectrolyte and a strong electrolyte. In all applications of the model the derived L-coefficients satisfy the well-known thermodynamic requirements. When appropriate ancillary data is available, the model yields fair to excellent predictions of the diffusion matrix at low concentrations of the solute species. Possible methods of improving the agreement at higher concentrations are discussed.